Synchronous motors may be of three different types which are the wound rotor (electromagnetic) type, the permanent magnet rotor type or the brushless type. However, in all types of synchronous motors, a rotating magnetic stator field produced by a set of stator windings causes the rotor to rotate in step or in synchronism with the stator field. In other words, the rotational frequency of the rotor equals the stator field frequency.
Essentially, synchronous motors operate by causing the magnetic poles of the rotor to be attracted to the revolving stator field. Thus, the magnetic poles of the rotor follow the revolving stator field in synchronism thereto, thereby producing a torque on the shaft of the rotor.
The torque angle of a synchronous motor is a measure of the angle between the applied stator voltage and the back EMF induced in the stator. The torque angle of the synchronous motor increases with increasing mechanical load and decreases with decreasing mechanical load. If a large mechanical load is applied to the motor, then the torque angle of the synchronous motor increases to a point where the motor stalls and synchronous operation is lost.
Thus, by maintaining the torque angle within a range of stability between zero and the angle at which the motor stalls, efficient synchronous operation of the motor is maintained. Typically, synchronous motors are designed to maintain a predetermined torque angle within the stability range at which the motor operates most efficiently considering such factors as expected load, cooling, desired speed, etc.
Mechanical load or load torques translate into a torque applied to the shaft of a rotor. Load torques are steady state in nature and if great enough can cause the torque angle to exceed the stability range of the synchronous motor thereby resulting in a loss of synchronous operation in the synchronous motor.
Transient torques result from fluctuations in the supply voltage or regions of instability existing in the torque speed envelope of the synchronous motor. The fluctuations in supply voltage or regions of instability cause the torque angle of the synchronous motor to increase to a point where synchronous operation of the synchronous motor is lost. Particularly, the rotor of a synchronous motor may, for example, begin to oscillate or hunt in an attempt to remain in synchronism with the stator field when sudden load torque changes occur. The oscillations may become so great that the torque angle of the synchronous motor may begin to exceed the range of stability of the synchronous motor, causing a loss of synchronous operation of the synchronous motor.
Various systems have been proposed for controlling and stabilizing the operation of a synchronous motor to minimize the effect of load torques and transient torques which cause the torque angle of a synchronous motor to vary from a predetermined angle within the stability range of the synchronous motor.
One conventional approach is to make use of a damper winding which is a shorted winding on the rotor. Damper windings tend to maintain motor synchronism by generating a torque in the rotor counteracting the applied load and transient torques. Damper windings tend to reduce the magnitude of any load or transient torques applied to the synchronous motor.
However, damper windings suffer from various disadvantages, namely, they increase the size and weight of the synchronous motor, and the damper windings reduce the efficiency of the synchronous motor, thereby consuming more power and making the synchronous motor harder to cool.
Another approach to eliminating the effects of load and transient torques on a synchronous motor is to provide an electronic synchronous motor control system for electronically controlling the synchronism of a damperless synchronous motor. Electronic synchronous motor control systems eliminate the use of damper windings because all necessary damping functions are performed electronically. An example of a conventional electronic synchronous motor control system is shown in FIG. 1.
The conventional system disclosed in FIG. 1 includes a voltage controlled oscillator 30, logic circuitry 32, a power inverter 34 and a synchronous motor 36.
As shown in FIG. 1, a speed command signal is supplied to the voltage controlled oscillator 30 which outputs a frequency signal related to the speed command signal. The frequency signal is applied to the logic circuitry 32 which outputs a pulse width modulated signal in response to the frequency signal and the speed command signal.
The pulse width modulated signal output by the logic circuitry 32 is provided to the power inverter 34 which outputs a variable voltage variable frequency signal for controlling the synchronous motor 36. The conventional system shown in FIG. 1 maintains synchronism of the synchronous motor by varying the voltage and frequency of the signal applied to the synchronous motor thereby maintaining the torque angle of the synchronous motor 36 at a predetermined value within the stability range.
Electronic synchronous motor control systems which control the synchronism of a damperless synchronous motor tend to be complex and expensive to construct. Further, such systems do not accurately detect the position of the rotor of the synchronous motor relative to the stator magnetic field in order to effectively reduce the effects of the transient torques on the torque angle of the motor.
Another conventional type of electronic synchronous motor control system controls synchronism of a damperless synchronous motor by using a shaft position sensor which is mounted on the rotor shaft to determine the position of the rotor flux or magnetic field. In this conventional system, a signal from the shaft position sensor controls the inverter frequency output by a power inverter. The signal from the shaft position sensor controls the inverter frequency stator MMF (magneto motive force) from getting too far ahead of the rotor MMF. Thus, the stator and rotor magnetic fields are maintained sufficiently close together to hold the torque angle within the stability range despite the presence of transient torques or sudden load torque changes.
Although electronic synchronous motor control systems for controlling the synchronism of a synchronous motor by using a shaft position sensor which provides a signal to a power inverter may accurately detect the position of the rotor relative to the stator field, the complexity and expense of such systems significantly increase.
Further examples of conventional electronic synchronous motor control systems are shown in U.S. Pat. Nos. 4,160,939, 4,511,834 and 4,629,958. U.S. Pat. No. 4,160,939 discloses a motor speed control system utilizing a counter for providing a count which is inversely proportional to the speed of the motor to thereby control a voltage controlled oscillator whose frequency is used for driving the motor. U.S. Pat. No. 4,511,834 stabilizes the operation of a damperless synchronous motor by developing a control voltage which is a function of the motor voltage for rapidly adjusting the inverter frequency which drives the motor. U.S. Pat. No. 4,629,958 discloses a synchronous motor control system which compensates the sensed rotational angle of the synchronous motor by the actual velocity of the synchronous motor to output a command signal to an inverter which provides a signal for controlling the synchronous motor.